Sterilization and Disinfection Practice Questions

Q: Autoclaving is not useful for disposal of which of the following items?

Liquid paraffin. **Explanation:** The core concept behind autoclaving is **moist heat sterilization**, which utilizes saturated steam under pressure. For autoclaving to be effective, the steam must come into direct contact with all surfaces of the material being sterilized. **Why Liquid Paraffin is the Correct Answer:** Liquid paraffin is an anhydrous (oily) substance. Oils, fats, and powders are **impermeable to steam**. Because steam cannot penetrate these substances, the latent heat of vaporization cannot be released internally to kill microorganisms. Therefore, liquid paraffin and other oily substances must be sterilized using **Dry Heat (Hot Air Oven)**, which relies on conduction rather than steam penetration. **Analysis of Incorrect Options:** * **A. Surgical Dressings:** These are porous materials. Steam easily penetrates the fibers of gauze and cotton, making autoclaving the gold standard for their sterilization. * **B. Metallic Instruments:** Stainless steel instruments are heat-stable and allow steam to condense on their surfaces, effectively destroying spores and vegetative cells. * **C. Petri Dishes:** If made of heat-resistant glass or specific plastics, these are routinely autoclaved in laboratory settings to ensure sterility before or after use. **High-Yield NEET-PG Pearls:** * **Standard Autoclave Conditions:** 121°C at 15 psi for 15–20 minutes. * **Sterilization Indicator:** *Geobacillus stearothermophilus* (formerly *Bacillus stearothermophilus*) spores are used as the biological indicator for autoclaving. * **Dry Heat (Hot Air Oven):** Used for "Glass, Oil, and Powder" (GOP). Standard cycle: 160°C for 2 hours. * **Biomedical Waste (BMW) Link:** Autoclaving is the preferred method for treating **Yellow (h) category** waste (microbiology waste) and **Red category** waste (contaminated plastics) before final disposal.

Q: Which of the following commonly used topical antiseptics has antifungal activity?

Povidone-Iodine. **Explanation:** **Povidone-Iodine (Option A)** is the correct answer because it is a broad-spectrum iodophor with microbicidal activity against bacteria (Gram-positive and Gram-negative), bacterial spores, viruses, and **fungi**. It works by releasing free iodine, which penetrates the cell wall and oxidizes key proteins, nucleotides, and fatty acids, leading to rapid cell death. Its antifungal efficacy makes it a preferred choice for treating fungal skin infections and preoperative skin preparation. **Why the other options are incorrect:** * **Cetrimide (Option B):** A quaternary ammonium compound (cationic detergent). While it is an excellent cleansing agent with activity against Gram-positive bacteria, it has **poor activity** against fungi and no activity against spores or viruses. * **Chlorhexidine (Option C):** A biguanide widely used for hand hygiene and surgical scrubbing. It is highly effective against Gram-positive bacteria but has **limited and inconsistent antifungal activity** (primarily fungistatic rather than fungicidal). * **Alcohol (Option D):** Ethyl or Isopropyl alcohol (60-90%) acts by denaturing proteins. While it is bactericidal and tuberculocidal, it is **not reliably effective against fungi** or bacterial spores. **High-Yield Clinical Pearls for NEET-PG:** * **Iodophors:** These are complexes of iodine with a solubilizing agent (like Povidone). They are less irritating and non-staining compared to the older Tincture of Iodine. * **Savlon:** A popular clinical combination of **Chlorhexidine and Cetrimide**, combining rapid action with detergent properties. * **Glutaraldehyde (2%):** Known as "Cidex," it is the agent of choice for "cold sterilization" of endoscopes (effective against spores and fungi). * **Skin Disinfection:** Chlorhexidine is often preferred over Povidone-iodine for central line insertion due to its persistent residual activity.

Q: Ethylene oxide (ETO) is used for the following EXCEPT:

Fumigation of rooms. Ethylene oxide (ETO) is a potent alkylating agent used for **gas sterilization** of heat-sensitive items. The correct answer is **Option B** because ETO is not used for room fumigation; instead, **Formaldehyde gas** or **Hydrogen Peroxide vapor** are the standard agents for disinfecting large spaces like Operation Theatres. ### Why Option B is Correct: ETO is highly explosive, flammable, and toxic (carcinogenic). Using it to fumigate an entire room is practically impossible and dangerous due to the risk of explosions and the difficulty in aerating the space to remove toxic residues. ### Why Other Options are Incorrect: * **Heart-Lung Machines (Option A):** ETO is the gold standard for complex medical machinery with electronic components and plastic tubing that would melt in an autoclave. * **Dental Equipment (Option B):** While many dental tools are autoclaved, ETO is used for specialized, heat-sensitive dental handpieces and plastic components. * **Clothing (Option D):** ETO has high penetrating power, making it suitable for sterilizing pre-packaged surgical kits, gowns, and bedding that cannot withstand steam. ### High-Yield Clinical Pearls for NEET-PG: * **Mechanism of Action:** Alkylation of amino, carboxyl, and hydroxyl groups in bacterial proteins and nucleic acids. * **Monitoring:** The biological indicator used to check ETO efficacy is **_Bacillus atrophaeus_** (formerly *B. subtilis var. niger*). * **Limitation:** Items sterilized by ETO require a long **aeration period** (8–12 hours) to remove residual gas, which can cause skin irritation or hemolysis. * **Alternative:** For room fumigation, **Formaldehyde** (generated from formalin and potassium permanganate) is the traditional choice, though increasingly replaced by **Fogging** with Hydrogen Peroxide.

Q: What is the recommended temperature and time period for sterilization using a hot air oven?

160 degrees Celsius for 2 hours. **Explanation:** Sterilization in a **Hot Air Oven** is the most common method of **dry heat sterilization**. It works primarily through **oxidative damage** to microbial proteins and electrolytes, as well as the toxic effects of elevated levels of electrolytes. **Why Option B is correct:** The standard, most widely accepted cycle for hot air oven sterilization is **160°C for 2 hours (120 minutes)**. This duration refers to the "holding time"—the period the load must remain at the target temperature after the oven has fully heated up. This time-temperature combination is sufficient to kill even the most resistant bacterial spores, such as those of *Clostridium tetani*. **Analysis of Incorrect Options:** * **Option A & D (140°C):** While 140°C is used in some protocols, it requires a much longer holding time (usually 3 hours) to achieve the same sterility assurance level as 160°C. * **Option C (160°C for 1 hour):** One hour at 160°C is generally considered insufficient for a guaranteed kill of all spores in a bulk load, increasing the risk of sterilization failure. **High-Yield NEET-PG Pearls:** * **Mechanism:** Death by oxidation of proteins. * **Sterilization Control (Biological Indicator):** Spores of ***Bacillus subtilis*** (var. *niger*) are used to check efficacy. * **What to Sterilize:** Glassware (Petri dishes, flasks, pipettes), surgical instruments (forceps, scalpels), and anhydrous materials like powders, fats, and oils. * **What NOT to Sterilize:** Heat-sensitive materials like rubber, plastics, or volatile liquids. * **Temperature-Time Variations:** 170°C for 1 hour or 180°C for 30 minutes are also recognized cycles, but 160°C for 2 hours remains the "gold standard" for exams.

Q: ULPA filters are used to disinfect which of the following?

Air. **Explanation:** **1. Why the Correct Answer is Right:** ULPA (**Ultra-Low Particulate Air**) filters are high-efficiency mechanical filters used specifically for the purification of **Air**. They are an advanced version of HEPA (High-Efficiency Particulate Air) filters. While HEPA filters remove 99.97% of particles ≥ 0.3 μm, ULPA filters are even more efficient, removing **99.999% of airborne particles ≥ 0.12 μm**. They work through a combination of interception, impaction, and diffusion to trap microbes, dust, and aerosols in specialized environments like cleanrooms and biosafety cabinets. **2. Why the Incorrect Options are Wrong:** * **Water (A):** Water is typically disinfected using physical methods like boiling or UV radiation, or chemical methods like chlorination and ozonation. Filtration of water requires membrane filters (e.g., 0.22 μm) to remove bacteria, but not ULPA filters. * **Urine (C) & Blood (D):** These are biological fluids. Disinfection or sterilization of these fluids (if required for disposal or lab use) involves autoclaving, chemical treatment (e.g., sodium hypochlorite for spills), or specialized liquid filtration. ULPA filters are structurally designed for gas/air flow and would clog instantly if used for viscous biological liquids. **3. High-Yield Clinical Pearls for NEET-PG:** * **HEPA vs. ULPA:** HEPA is the standard for Operating Theaters (OT) and Laminar Air Flow benches (removes particles ≥ 0.3 μm). ULPA is used where higher sterility is required (e.g., pharmaceutical manufacturing). * **Efficiency:** ULPA filters are rated at 99.999% efficiency (often called "five nines"). * **Application:** Essential in **Biosafety Cabinets (BSC)** to protect the laboratory worker and the environment from infectious aerosols. * **Note:** Filters (HEPA/ULPA) do **not** kill microbes; they physically remove them, making this a method of **disinfection/sterilization by filtration**.

Q: The spores of which bacterium are used to check the efficacy of hot air ovens?

Bacillus subtilis. **Explanation:** Sterilization monitoring is a high-yield topic in NEET-PG. The efficacy of sterilization methods is verified using **Biological Indicators**, which utilize the most resistant spores of specific non-pathogenic bacteria. **Why Bacillus subtilis is correct:** Hot air ovens utilize **dry heat** to achieve sterilization. The spores of **_Bacillus subtilis_ (subspecies _niger_)**, also known as **_Bacillus atrophaeus_** in modern taxonomy, are the gold standard for dry heat monitoring. These spores are highly resistant to desiccation and high temperatures, making them ideal for testing the efficiency of hot air ovens and ethylene oxide (EtO) sterilization. **Analysis of Incorrect Options:** * **A. Bacillus stearothermophilus:** These spores are highly resistant to **moist heat**. Therefore, they are the biological indicator of choice for **Autoclaves** and plasma sterilization. * **B. Bacillus atrophaeus:** While taxonomically identical to _B. subtilis var. niger_, in many traditional microbiology textbooks and NEET-PG patterns, **_Bacillus subtilis_** remains the preferred nomenclature for the dry heat indicator. * **D. Bacillus cereus:** This is a common cause of food poisoning (reheated rice syndrome) and is not used as a standard biological indicator for sterilization. **Clinical Pearls for NEET-PG:** * **Autoclave (Moist Heat):** _Geobacillus stearothermophilus_ (formerly _B. stearothermophilus_). * **Hot Air Oven (Dry Heat):** _Bacillus subtilis_ (or _B. atrophaeus_). * **Ionizing Radiation:** _Bacillus pumilus_. * **Ethylene Oxide:** _Bacillus subtilis_ (var. _niger_). * **Filtration:** _Brevundimonas diminuta_. * **Incubation:** After the sterilization cycle, biological indicators are incubated at 55-60°C (for _stearothermophilus_) or 37°C (for _subtilis_) to check for growth. No growth indicates successful sterilization.

Q: Which of the following is NOT a sporicidal disinfectant?

Benzalkonium chloride. **Explanation:** The core concept tested here is the classification of disinfectants based on their **biocidal activity**. Disinfectants are categorized as high, intermediate, or low-level based on their ability to kill resistant microorganisms like bacterial spores. **Why Benzalkonium chloride is the correct answer:** Benzalkonium chloride is a **Quaternary Ammonium Compound (QAC)**, which acts as a **low-level disinfectant**. It works by denaturing cell membrane proteins and disrupting lipid bilayers. While effective against most vegetative bacteria, fungi, and enveloped viruses, it is **not sporicidal**, tuberculocidal, or effective against non-enveloped viruses. **Analysis of incorrect options:** * **Glutaraldehyde (2%):** Known as a "cold sterilant," it is a high-level disinfectant that is **sporicidal** (requires prolonged contact time, usually 3–10 hours). It acts by alkylation of amino and hydroxyl groups. * **Formaldehyde:** A high-level disinfectant and gaseous sterilant. It is **sporicidal** but rarely used for instruments due to its toxicity and pungency; it is primarily used for fumigation and preserving specimens. * **Ethylene oxide (EtO):** A potent gaseous sterilant used for heat-sensitive items. It is highly **sporicidal** through the process of alkylation. **High-Yield Clinical Pearls for NEET-PG:** * **Sporicidal Agents (High-level):** Glutaraldehyde, Formaldehyde, Ethylene oxide, Hydrogen peroxide (6-30%), and Peracetic acid. * **Low-level Disinfectants:** Quaternary ammonium compounds (Benzalkonium chloride) and Chlorhexidine (Biguanides). * **Cidex:** The commercial name for 2% Glutaraldehyde. * **Surface Disinfection:** For COVID-19 or general surface disinfection in hospitals, 1% Sodium Hypochlorite is the standard. * **Skin Antiseptic:** 70% Isopropyl alcohol is the most common, but it is not sporicidal.

Q: Which of the following is considered the best antiseptic?

Betadine. **Explanation:** **Betadine (Povidone-iodine)** is considered the best and most widely used antiseptic in clinical practice because it is an **iodophor**. It provides a sustained release of free iodine, which acts as a powerful oxidizing agent, denaturing proteins and nucleic acids of microorganisms. Its superiority lies in its **broad spectrum of activity** (effective against bacteria, spores, fungi, protozoa, and viruses) and its **residual effect** on the skin. Unlike elemental iodine, it is non-staining and significantly less irritating to tissues. **Analysis of Incorrect Options:** * **Alcohol (Ethyl/Isopropyl):** While rapid-acting and excellent for skin pricks, it lacks sporicidal activity and has no residual effect because it evaporates quickly. It also cannot be used on open wounds as it causes tissue protein coagulation (stinging/irritation). * **Savlon:** A combination of Chlorhexidine and Cetrimide. While a good detergent and antiseptic, it is less effective against Pseudomonas and certain viruses compared to Betadine. * **Phenol:** Historically the "gold standard" (Phenol Coefficient), it is now rarely used as an antiseptic because it is highly corrosive to tissues and potentially toxic. It is primarily used as a disinfectant for inanimate objects or in dilute forms as a preservative. **High-Yield Clinical Pearls for NEET-PG:** * **Pre-operative Gold Standard:** Povidone-iodine (7.5%–10%) is the preferred agent for surgical site preparation. * **Contact Time:** Iodine requires at least **2 minutes** of contact time to be fully effective. * **Contraindication:** Avoid in patients with iodine hypersensitivity or significant thyroid dysfunction. * **Sporicidal Activity:** Iodine is one of the few antiseptics that can be sporicidal with prolonged contact time.

Q: A chest physician is performing bronchoscopy in the procedure room of the outpatient department using the instrument shown below. What is the most appropriate method to disinfect the instrument to make it safe for use in the next patient?

2% glutaraldehyde for 20 minutes. ***2% glutaraldehyde for 20 minutes*** - **Bronchoscopes** are classified as **semi-critical instruments** per Spaulding classification, requiring **high-level disinfection** as they contact mucous membranes but don't penetrate sterile tissues. - **2% glutaraldehyde** for **20 minutes** is the **gold standard** for high-level disinfection of flexible endoscopes, effectively eliminating **vegetative bacteria**, **mycobacteria**, **viruses**, and **most spores**. *70% alcohol for 5 minutes* - **Alcohol** provides only **intermediate-level disinfection** and is insufficient for **semi-critical instruments** like bronchoscopes. - The **5-minute contact time** is inadequate for eliminating resistant organisms like **Mycobacterium tuberculosis** and **hepatitis viruses**. *2% formaldehyde for 10 minutes* - **Formaldehyde** can damage the **delicate fiber-optic components** and **rubber seals** of flexible bronchoscopes. - The **10-minute exposure** is insufficient for achieving **high-level disinfection** required for semi-critical medical devices. *1% sodium hypochlorite for 15 minutes* - **Sodium hypochlorite (bleach)** is **highly corrosive** and can cause permanent damage to the **metal components** and **optical fibers** of bronchoscopes. - While effective as a disinfectant, it's **contraindicated** for use on delicate endoscopic equipment due to its corrosive properties.

Question 1

Autoclaving is not useful for disposal of which of the following items?

Accepted Answer

Liquid paraffin

Answer

Surgical dressings

Answer

Metallic instruments

Answer

Petri dishes

Question 2

Which of the following commonly used topical antiseptics has antifungal activity?

Accepted Answer

Povidone-Iodine

Answer

Cetrimide

Answer

Chlorhexidine

Answer

Alcohol

Question 3

Ethylene oxide (ETO) is used for the following EXCEPT:

Accepted Answer

Fumigation of rooms

Answer

Sterilization of Heart-Lung Machine

Answer

Sterilization of Dental equipments

Answer

Sterilization of clothing

Question 4

What is the recommended temperature and time period for sterilization using a hot air oven?

Accepted Answer

160 degrees Celsius for 2 hours

Answer

140 degrees Celsius for 1 hour

Answer

160 degrees Celsius for 1 hour

Answer

140 degrees Celsius for 2 hours

Question 5

ULPA filters are used to disinfect which of the following?

Accepted Answer

Air

Answer

Water

Answer

Urine

Answer

Blood

Question 6

The spores of which bacterium are used to check the efficacy of hot air ovens?

Accepted Answer

Bacillus subtilis

Answer

Bacillus stearothermophilus

Answer

Bacillus atrophaeus

Answer

Bacillus cereus

Question 7

Endoscope washing with deionized water is associated with which organism?

Accepted Answer

Mycobacterium chelonae

Answer

Mycobacterium tuberculosis

Answer

Mycobacterium bovis

Answer

Mycobacterium ulcerans

Question 8

Which of the following is NOT a sporicidal disinfectant?

Accepted Answer

Benzalkonium chloride

Answer

Glutaraldehyde

Answer

Formaldehyde

Answer

Ethylene oxide

Question 9

Which of the following is considered the best antiseptic?

Accepted Answer

Betadine

Answer

Alcohol

Answer

Savlon

Answer

Phenol

Question 10

A chest physician is performing bronchoscopy in the procedure room of the outpatient department using the instrument shown below. What is the most appropriate method to disinfect the instrument to make it safe for use in the next patient?

Accepted Answer

2% glutaraldehyde for 20 minutes

Answer

70% alcohol for 5 minutes

Answer

2% formaldehyde for 10 minutes

Answer

1% sodium hypochlorite for 15 minutes

Sterilization and Disinfection — MCQs

Sterilization and Disinfection — MCQs

On this page

Practice by Chapter

Want unlimited practice?